A digital image processing method has been proposed wherein a random sample of tone values from the informational portion of the digital image is normalized to produce a tone reproduction function. The tone reproduction function is then applied to the digital image to produce a processed digital image. This procedure was frst proposed in the article entitled "A New Approach to Programming in Photomechanical Reproduction" by Yu. Ovchinnikov et al, 12th IARIGAI Conference Proceedings, Versailles, France, Editor W. Banks, IPC Science and Technology Press, Guildford, England, 1974, pages 160-163. It has been discovered that the success of the method depends strongly upon choosing a truly random sample of tone values from the image. An improvement to the randomness of the sampling by selecting the tone values from a "floating" image dependent contrast interval having certain preferred statistical properties is disclosed in copending U.S. patent application Ser. No. 730,630. A further improvement wherein the overall contrast of the image is adjusted as a function of the standard deviation of the tone values of the image and the tone reproduction function is expressed in terms of the standard normal variate is disclosed in copending U.S. patent application Ser. No. 730,629.
An extension of the method to processing digital color images to adjust both tone-scale and color balance is disclosed in copending U.S. patent application Ser. No. 730,627. In the extension to digital color images a color reproduction function is produced for each of a plurality of color components (e.g., red, green and blue) in the digital image by normalizing respective samples of color values selected from the informational portions of the image. The color reproduction functions thus produced are then applied to the respective color components of the digital color image to produce the processed digital color image. An output device is calibrated so that combinations of color values having equal values of the standard normal variate in the color reproduction functions produce a neutral (gray) color.
This method generally produces an aesthetic improvement in the appearance of processed color images. However, when the digital color image is derived from a color photographic film image containing highly saturated textures such as large areas of green grass or blue sky noise, the randomness of the sampling procedure for producing the samples of color values for normalization is subject to influence, thereby distorting the color reproduction functions and causing unwanted color shifts in the processed images.
It is apparent from the examination of such processed images that further improvement in the digital color image processing method is desirable.
It is the object of the present invention to provide an improved digital color image processing method of the type described above wherein the appearance of processed images having highly saturated textured areas such as green grass and blue sky noise are improved.